Ni-plated steel sheet for cans and method for producing same

Abstract

The invention provides a Ni-plated steel sheet for cans and a method for manufacturing the same. The Ni-plated steel sheet for cans according to the present invention contains, in wt%, 0.020-0.060% of C, 0.050% or less of Si, 0.1-0.6% of Mn, 0.010-0.060% of Al, 0.015% or less of P, 0.015% or less of S, 0.006% or less of N, less than 0.030% of Cr, and the balance of Fe and other unavoidable impurities, has a component index represented by relational expression 1 of 0.85 or less, has an ASTM crystal grain size number of 11.0-11.7, and has a steel microstructure containing a carbide having an equivalent circle average size of 1.0-2.0 [mu] m, an Fe-Ni alloy layer having a thickness of 0.6-1.4 [mu] m is formed on the surface of the Ni-plated steel sheet for cans.

Description

Technical Field

[0001] The present invention relates to a Ni-plated steel sheet having excellent strength and workability and a method for manufacturing the same, and more particularly to a Ni-plated steel sheet having excellent strength and workability for use in an electric vehicle battery case or the like and a method for manufacturing the same. Background Art

[0002] In the case of a cylindrical can for use in a cylindrical battery case, a method of nickel (Ni)-plating a steel sheet is generally used to resist corrosion caused by the entry of an electrolyte into the battery contents. In recent years, with the increasing demand for electric vehicles, the demand for materials for cylindrical battery cases for electric vehicles has increased significantly.

[0003] In addition, in order to ensure battery safety at high temperatures, the requirement for the strength of the battery case material at high temperatures is increasing. When the battery generates heat due to factors such as overcurrent, an abnormal chemical reaction is caused, and the temperature may rise to about 600 °C. Therefore, the battery case material may be instantaneously exposed to a high temperature of several hundred degrees (°C), and thus heat resistance characteristics must be ensured to maintain the shape at high temperatures. When the vehicle is running, the battery case may be deformed due to a temperature rise, which may affect the running of the vehicle. Therefore, in order to prevent this, deformation at high temperatures must be strictly suppressed.

[0004] The heat resistance characteristics of an electric vehicle battery can be evaluated by various methods. As an example, the stability of a battery (Cell) is evaluated by heating the electric vehicle battery to a temperature of about 600 °C. In order to prevent deformation of the battery, the yield strength of the battery case material at 600 °C must satisfy 80 MPa or more.

[0005] In addition to the physical properties at high temperatures described above, the physical properties of the battery case material at normal temperature are also important. When the strength at normal temperature is low, sagging may occur after filling the battery contents. Therefore, a yield strength above a certain level is required to prevent this.

[0006] Furthermore, normal temperature physical properties are also required in terms of the workability of the material. When forming a cylindrical battery case, multi-stage processing techniques such as drawing and ironing are required. Therefore, in addition to the above-mentioned high temperature characteristics, processing characteristics at normal temperature are also required. When the yield strength is higher than a certain level, there is a problem that serious die wear occurs during processing and the die needs to be frequently replaced. Therefore, in order to avoid this, a high elongation rate above a certain level is also required in order to form the can into a desired shape without generating cracks.

[0007] Cylindrical battery cases are usually nickel-plated to prevent corrosion by the internal electrolyte or the atmosphere during the manufacturing process, and the plating also has required characteristics. In the case of nickel plating, the plating is usually performed with a thickness of 1 μm or more. To prevent the plating from peeling off during processing, heat treatment is carried out to form an Fe-Ni alloy layer at the interface by diffusion between the Fe in the steel sheet and the Ni in the plating, thereby improving the adhesion. When the thickness of the alloy layer is too thin, it is difficult to ensure the adhesion between the steel sheet and the plating. When the thickness of the alloy layer is too thick, the Fe component is exposed to the surface of the plating, and rusting may occur due to the oxidation of Fe. Therefore, it is necessary to form an alloy layer with an appropriate thickness.

[0008] Korean Patent Publication No. 2019-0078406 discloses a method of adding Nb, Cr, W, etc. to a super-low-carbon steel with C of 0.003% or less and using fine precipitates to ensure room-temperature and high-temperature properties. Although the properties can be improved by fine precipitates, expensive precipitate-forming elements need to be added, and there is a disadvantage of an increased load on the steelmaking process for manufacturing super-low-carbon steel.

[0009] Korean Patent Publication No. 1999-0053991 discloses a method of manufacturing a high-strength steel sheet for cans by subjecting a normal low-carbon steel with C at a level of 0.04 - 0.06% to secondary rolling. As the most common method for increasing the strength of can materials, it is characterized in that secondary reduction is carried out at 20 - 30% after recrystallization annealing, so that the advantage of significantly increasing the strength by work hardening can be obtained. However, when the reduction is at the above-mentioned high level, the elongation rate is significantly reduced, so there is a disadvantage of difficulty in ensuring can processability.

[0010] Korean Patent Publication No. 2018-0109964 discloses a method that can improve the strength by solid-solution strengthening with the addition of a large amount of N of 130 ppm or more and apply a low secondary reduction rate of 20% or less to improve the elongation rate. However, when a large amount of N, which is an interstitial element, is added, composition deviation may easily occur, and when composition deviation occurs, the possibility of material deviation is high. Therefore, in order to control the composition deviation at a low level, there is a disadvantage of the need for additional efforts during the steelmaking process.

[0011] Korean Patent Publication No. 2021-0091795 discloses the following method: ensuring the strength by precipitation strengthening with the addition of Ti, reducing the decrease in elongation rate caused by work hardening by further applying a relatively lower secondary reduction rate of 15% or less, and ensuring the balance between strength and ductility. However, the addition of Ti has the following characteristics: due to the high oxygen affinity of Ti, a large amount of inclusions are formed in the steelmaking process, so the cleanliness is reduced. When there are many inclusions in the steel, they may become the starting points of cracks during the forming process, so there is a disadvantage of the need for additional efforts to remove the inclusions. Summary of the Invention

[0012] (I) Technical Problem to be Solved

[0013] The object of the present invention is to manufacture a Ni-plated steel sheet that has both excellent strength and workability and can be used as a material for an electric vehicle battery case.

[0014] In addition, the technical problems to be solved by the present invention are not limited to the above technical problems, and those skilled in the art can clearly understand other technical problems not mentioned from the following description.

[0015] (II) Technical Solution

[0016] One aspect of the present invention relates to a Ni-plated steel sheet for cans. In terms of weight %, the Ni-plated steel sheet for cans contains: C: 0.020 - 0.060%, Si: 0.050% or less, Mn: 0.1 - 0.6%, Al: 0.010 - 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.006% or less, Cr: less than 0.030%, the balance being Fe and other inevitable impurities, the component index represented by the following relational expression 1 is 0.85 or less, the ASTM grain size number satisfies 11.00 to 11.70, the Ni-plated steel sheet for cans has a fine steel structure containing carbides with an equivalent circle average size satisfying 1.0 - 2.0 μm, and an Fe-Ni alloy layer with a thickness of 0.6 - 1.4 μm is formed on the surface of the Ni-plated steel sheet for cans.

[0017] [Relational Expression 1]

[0018] Component index = C × 10 + Mn

[0019] Wherein, C and Mn respectively represent the weight % of C and Mn.

[0020] The normal temperature yield strength of the Ni-plated steel sheet for cans can satisfy 320 - 400 MPa, the normal temperature elongation can satisfy 30% or more, and the yield strength at 600 °C can satisfy 80 MPa or more.

[0021] In addition, another aspect of the present invention relates to a method for manufacturing a Ni-plated steel sheet for cans, the manufacturing method comprising the following steps: reheating a steel slab satisfying the above composition and relational expression 1 to a temperature above 1150°C; hot finish rolling the reheated steel slab above Ar3 to manufacture a hot-rolled steel sheet; after cooling the hot-rolled steel sheet, coiling it at a temperature of 580 - 720°C; cold rolling the coiled hot-rolled steel sheet at a reduction ratio of 78 - 90% to manufacture a first cold-rolled steel sheet; annealing the first cold-rolled steel sheet at a temperature of 630 - 810°C; cold rolling the annealed steel sheet at a reduction ratio of 0.6 - 1.2% to manufacture a second cold-rolled steel sheet; subjecting the second cold-rolled steel sheet to Ni electroplating to manufacture a Ni-plated steel sheet; and subjecting the Ni-plated steel sheet to alloying annealing at a temperature of 650 - 700°C for a time when the alloying index defined in the following relational expression 2 satisfies 2.10 to 2.60, thereby forming an Fe-Ni alloy layer with a thickness of 0.6 - 1.4 μm on the surface of the cold-rolled steel sheet.

[0022] [Relational expression 1]

[0023] Composition index = C×10 + Mn

[0024] Wherein, C and Mn respectively represent the weight % of C and Mn.

[0025] [Relational expression 2]

[0026] Alloying index = (2 - (700 - T) / 100)×t 0.1

[0027] Wherein, T represents the temperature (°C), and t represents the time (seconds).

[0028] During the Ni electroplating, it is preferably carried out with a Ni plating thickness in the range of 0.6 - 5.0 μm.

[0029] (III) Beneficial effects

[0030] According to the present invention, a Ni-plated steel sheet can be provided, the Ni-plated steel sheet having a room temperature yield strength of 320 - 400 MPa, a room temperature elongation of 30% or more, a yield strength of 80 MPa or more at 600°C, the Fe-Ni alloy layer having a thickness of 0.6 - 1.4 μm, and being applicable to cylindrical battery casings. Best mode for carrying out the invention

[0031] Hereinafter, the present invention will be described.

[0032] The present invention relates to a Ni-plated steel sheet for use in an electric vehicle battery case, etc. after can forming. Materials for corresponding applications need to have a strength above an appropriate level to maintain their shape at normal and high temperatures, and in order to ensure workability, considering the decrease in elongation rate when increasing strength, it is necessary to optimize the balance between strength and elongation rate. In addition, for the Ni coating, it is necessary to ensure a corrosion resistance and adhesion above a certain level. The corrosion resistance is used to prevent corrosion, and the adhesion is used to prevent peeling during processing.

[0033] Therefore, in order to achieve the above object, the inventors of the present invention conducted repeated research and experiments, and as a result, it was found that a plated steel sheet having the above target physical properties can be manufactured by optimizing the types and contents of alloying elements and manufacturing conditions, and thus the present invention was proposed.

[0034] Hereinafter, first, the composition of the cold-rolled steel sheet provided by the present invention will be described in detail. At this time, unless otherwise specifically stated, the content of each component is expressed as weight %.

[0035] Carbon (C): 0.020 - 0.060%

[0036] Carbon (C) is an element added to improve the strength of the steel sheet. When the content of carbon (C) is low, the strength is low and it is difficult to be used as a structural member, so a content of 0.020% or more is preferred. In addition, in order to reduce the content to less than 0.020%, the load of the steelmaking process increases, and thus the productivity decreases. On the other hand, when the content of carbon (C) is too high, the strength is too high, which will increase the die wear rate during forming, reduce the elongation rate, and reduce the formability. Therefore, it is preferably controlled below 0.060%. More preferably, it is controlled in the range of 0.030 - 0.050%.

[0037] Silicon (Si): 0.050% or less

[0038] Si is an element that can be used as a deoxidizer and can help improve strength through solid solution strengthening, so it is difficult to completely eliminate Si. However, when the Si is too high, Si-based oxides are generated on the surface during annealing, which may cause defects during plating and reduce the plating property. Therefore, considering this point, the upper limit is preferably set at 0.050% or less. More preferably, it is controlled below 0.030%.

[0039] Manganese (Mn): 0.1 - 0.6%

[0040] Mn combines with the dissolved S in the steel and precipitates in the form of MnS, and it is an element that prevents the hot shortness caused by the dissolved S. To achieve this effect, it is preferable to contain more than 0.1% of Mn. In addition, the Mn, together with the C dissolved in the steel, also has the effect of increasing the strength of the steel. However, when the Mn is in excess, the workability of the steel decreases, so the content of the Mn is preferably 0.6% or less.

[0041] Aluminum (Al): 0.010 - 0.060%

[0042] Al is an element with a very large deoxidation effect. It reacts with the N in the steel to precipitate AlN, thereby preventing the reduction of formability caused by the dissolved N. To obtain the above effect, it is necessary to add more than 0.010% of Al. However, when added in large amounts, the effect brought by the additional addition is very small, so the content of Al is limited to 0.060% or less.

[0043] Phosphorus (P): 0.015% or less

[0044] When adding P below a certain amount, the ductility of the steel will not be significantly reduced, and it is an element that can increase the strength. However, when the addition amount of the phosphorus (P) exceeds 0.015%, it segregates at the grain boundaries, over-solidifies the steel, and reduces the elongation. Therefore, it is preferably limited to 0.015% or less.

[0045] Sulfur (S): 0.015% or less

[0046] S is an element that causes hot shortness during solid solution, so it is necessary to induce the precipitation of MnS by adding Mn. As the content of S increases, it is necessary to further add an appropriate level of Mn. Therefore, it is not preferable to have a large amount of S. Therefore, the upper limit of S is limited to 0.015%.

[0047] Nitrogen (N): 0.006% or less

[0048] N is an inevitable element remaining in the steel, but N existing in the dissolved state causes aging, so the workability is significantly reduced. To minimize the reduction of ductility caused by the occurrence of unnecessary levels of aging, the upper limit of N is preferably limited to 0.006% or less.

[0049] Chromium (Cr): less than 0.030%

[0050] Cr is an element that can improve workability when added in small amounts and is often added to materials for processing. However, when added, it accumulates on the surface and forms Cr-based oxides, which may reduce conductivity, and surface defects may occur due to non-plating during electroplating. In particular, when the coating is thin and the thickness is less than a few μm, since the influence on the surface is significant, the content of Cr is preferably limited to less than 0.030%. More preferably, it is controlled below 0.020%.

[0051] The component index represented by the following relational expression 1 is 0.85 or less

[0052] The present inventors defined and introduced a component index as shown in the following relational expression 1, and the component index is an index having strength and correlation increased by C and Mn. When the component index defined by the following relational expression 1 exceeds 0.85, it may be difficult to process due to excessive yield strength.

[0053] [Relational expression 1]

[0054] Component index = C × 10 + Mn (where C and Mn in the formula represent the weight percentages of C and Mn, respectively)

[0055] In addition to the above components, it preferably contains the balance of Fe and other inevitable impurities, and the addition of other components to the steel of the present invention is not excluded. These inevitable impurities may be inadvertently mixed in from raw materials or the surrounding environment during the normal steel manufacturing process, so these impurities cannot be excluded. Those skilled in the art of normal steel manufacturing can understand the inevitable impurities.

[0056] Next, the microstructure of the cold-rolled steel sheet of the present invention will be described.

[0057] The cold-rolled steel sheet of the present invention has a mixed structure of ferrite and cementite. In terms of area%, the structure fraction preferably contains 90% or more of ferrite and 10% or less of cementite. When the cementite is excessive, cracks are likely to occur during processing, so the workability will be reduced.

[0058] The ASTM grain size number of the microstructure of the cold-rolled steel sheet of the present invention satisfies 11.00 to 11.70.

[0059] In addition, the microstructure of the cold-rolled steel sheet of the present invention may contain carbides with an equivalent circle average size satisfying 1.0 - 2.0 μm. Generally, carbon (C) can exist in the form of combining with Fe to form carbides. When the size of the carbides is small, burrs are likely to be generated during shearing in the processing, and the shearing cross-sectional shape may be poor. When the size of the carbides is large, cracks may be caused during forming. Considering this, in the present invention, the equivalent circle average size of the carbides is preferably limited to the range of 1.0 - 2.0 μm.

[0060] In addition, an Fe-Ni alloy layer with a thickness of 0.6-1.4 μm is formed on the surface of the cold-rolled steel sheet of the present invention. When the thickness of the Fe-Ni alloy layer is too thin, it may be difficult to ensure plating adhesion. When the thickness of the Fe-Ni alloy layer is too thick, the Fe component contained in the steel sheet is exposed to the surface of the coating, so it may be difficult to ensure corrosion resistance.

[0061] The room temperature yield strength of the Ni-plated steel sheet of the present invention having the alloy composition and microstructure as described above satisfies 320-400 MPa, the room temperature elongation satisfies 30% or more, and the yield strength at 600 °C satisfies 80 MPa or more, so that it can be effectively used as a Ni-plated steel sheet for cans.

[0062] Next, a method for manufacturing a Ni-plated steel sheet according to an embodiment of the present invention will be described in detail.

[0063] The method for manufacturing a Ni-plated steel sheet for cans of the present invention includes the following steps: reheating a steel slab satisfying the above composition and relational expression 1 to 1150 °C or higher; hot finish rolling the reheated steel slab at a temperature above Ar3 to manufacture a hot-rolled steel sheet; after cooling the hot-rolled steel sheet, coiling it at a temperature of 580-720 °C; subjecting the coiled hot-rolled steel sheet to first cold rolling with a reduction rate of 78-90% to manufacture a first cold-rolled steel sheet; annealing the first cold-rolled steel sheet at a temperature of 630-810 °C; subjecting the annealed steel sheet to second cold rolling with a reduction rate of 0.6-1.2% to manufacture a second cold-rolled steel sheet; Ni electroplating the second cold-rolled steel sheet to manufacture a Ni-plated steel sheet; and subjecting the Ni-plated steel sheet to alloying annealing at a temperature of 650-700 °C for a time when the alloying index defined in relational expression 2 satisfies 2.1 to 2.6, so as to form an Fe-Ni alloy layer with a thickness of 0.6-1.4 μm on the surface of the cold-rolled steel sheet.

[0064] Slab reheating

[0065] First, in the present invention, the steel slab is reheated to a temperature of 1150 °C or higher. Since various precipitates generated in the steel must be redissolved when manufacturing the slab, the temperature is preferably 1150 °C or higher. Preferably, it can be reheated to the range of 1150-1300 °C.

[0066] Hot rolling

[0067] Next, in the present invention, the reheated slab is hot finish rolled at a temperature above Ar3 to manufacture a hot-rolled steel sheet. The reason for limiting the hot finish rolling temperature to above Ar3 is to perform rolling in the austenite single-phase region. When rolling in the two-phase region, the rolling stability may be reduced due to non-uniform material.

[0068] Coiling of hot-rolled steel sheet

[0069] In addition, in the present invention, after cooling the hot-rolled steel sheet, coiling is performed at 580 - 720°C. According to the coiling temperature, the size of the grains of the hot-rolled steel sheet changes. When the temperature is low, fine grains are formed, and when the temperature is high, coarse grains are formed. In addition, when the coiling temperature is too high, coarse carbides are formed, which may cause cracks during can processing. In order to obtain grains and carbide sizes suitable for strength and workability, the coiling temperature of the hot-rolled steel sheet is preferably controlled at 580 - 720°C.

[0070] Cold rolling of hot-rolled sheet

[0071] Next, in the present invention, the coiled hot-rolled steel sheet is cold-rolled once at a reduction ratio of 78 - 90% to manufacture a cold-rolled steel sheet for the first time. The reduction ratio of the first cold rolling is important for improving the strength at high temperatures. As the reduction ratio increases, recrystallization nucleation during annealing proceeds smoothly, refining the grains, and the high-temperature strength has a tendency to increase. In order to obtain a sufficient level of high-temperature strength, it is preferable to apply a reduction ratio of more than 78%. However, when the cold rolling reduction ratio exceeds 90%, the deformation resistance caused by rolling increases excessively, making rolling difficult, and the shape after rolling may be poor. Considering this, the cold rolling reduction ratio is preferably set in the range of 78 - 90%. By adding a pickling process before cold rolling, the scale generated during hot rolling can be removed.

[0072] Annealing of cold-rolled steel sheet for the first time

[0073] In addition, in the present invention, the cold-rolled steel sheet for the first time is annealed at a temperature of 630 - 810°C to manufacture an annealed steel sheet. The main purpose of annealing the cold-rolled steel sheet is to remove the internal stress formed during cold rolling and ensure workability. For this purpose, it is necessary to perform at a sufficiently high temperature to cause complete recrystallization. In order to cause recrystallization of the cold-rolled steel sheet having the composition system of the present invention, a temperature of 630°C or higher is required. However, when the temperature exceeds 810°C, it is difficult to ensure the strength at high temperatures, and due to the reduction in strength during the annealing process, plate fracture defects may be caused, so it is preferably controlled below this temperature.

[0074] Second rolling of annealed steel sheet

[0075] Next, the annealed steel sheet is cold-rolled at a reduction ratio of 0.8 - 1.8% to manufacture a cold-rolled steel sheet for the second time. Since the annealed steel sheet has usually a poor shape due to passing through a high temperature, in order to improve the shape and impart a desired roughness, it is preferable to perform the second cold rolling at a reduction ratio of 0.8 - 1.8%. When the cold rolling reduction ratio is as low as less than 0.8%, it is difficult to obtain the effect of improving the shape and imparting roughness, and when the cold rolling reduction ratio exceeds 1.8%, the strength has a tendency to increase above the desired level.

[0076] Ni Plating of Double Cold Rolled Steel Sheet

[0077] In addition, in the present invention, the double cold rolled steel sheet is Ni electroplated to manufacture a plated steel sheet. In order to ensure corrosion resistance to battery electrolyte and the atmosphere, Ni plating is preferably carried out. The plating thickness can vary depending on the forming amount, the type of electrolyte, etc. Considering roughness, it is preferably plated in a thickness range of 0.6 - 5.0 μm. In the case of hot dip plating, it is difficult to control the plating thickness below a certain thickness, and the thickness deviation tends to become large, which is not suitable for use as a cylindrical battery case material.

[0078] Alloying Annealing of Plated Steel Sheet

[0079] Subsequently, in the present invention, the plated steel sheet is annealed to manufacture an alloying annealed steel sheet. The adhesion of the Ni coating to the steel sheet after electroplating is not excellent, so it may be easily peeled off during processing. To prevent this, annealing at a high temperature is required to form an Fe-Ni alloy layer between the Ni coating and the steel sheet by diffusion. When the alloying annealing temperature is low, since the Fe-Ni alloy layer is thin, it is difficult to ensure adhesion, so annealing is preferably carried out at a temperature above a certain level. On the other hand, when the annealing temperature is high, the alloy layer becomes thick, and the Fe component contained in the steel sheet is exposed to the surface of the coating, making it difficult to ensure corrosion resistance, so annealing is preferably carried out at a temperature below a certain level.

[0080] The inventors found that the thickness of the alloy layer that can simultaneously ensure adhesion and corrosion resistance is 0.6 - 1.4 μm. For this purpose, alloying annealing is preferably carried out at a temperature of 650 - 700 °C, and the alloying annealing time is the time when the alloying index defined by the following relational expression 2 satisfies 2.1 to 2.6. The inventors found that there is a correlation between the alloying index, which has a linear relationship with temperature and an exponential relationship with time, and the thickness of the alloy layer, and established a formula for the alloying index. When the alloying index is in the range of 2.10 to 2.60, an alloy layer with a desired thickness can be formed.

[0081] [Relational Expression 2]

[0082] Alloying index = (2 - (700 - T) / 100) × t 0.1

[0083] Wherein, T represents the temperature (°C), and t represents the time (seconds).

[0084] However, the correlation among the alloying temperature, time, and alloy layer thickness is limited to steel plates manufactured under the composition and manufacturing conditions described in the present invention, and may not be applicable to steel plates with different compositions and manufacturing processes. As an example, when recrystallization annealing is not performed after the first cold rolling and recrystallization and alloying occur simultaneously during alloying annealing, the diffusion rate at the interface between Fe and Ni is relatively fast, so the thickness of the alloy layer tends to be too thick. Detailed Description of the Invention

[0085] Hereinafter, the present invention will be described in more detail by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention for more detailed description and are not used to limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the content recorded in the claims and the content reasonably deduced therefrom.

[0086] (Examples)

[0087] Manufacture steel plates having the composition shown in Table 1 below and the manufacturing conditions shown in Table 2 below. The components are marked in the form of actual values, and each slab with the corresponding composition is manufactured. The slab is reheated to 1220 °C and then hot-rolled at a thickness of 4 mm at a temperature above 900 °C and coiled at the temperature shown in Table 2 below. The hot-rolled steel plate after coiling is cold-rolled at the reduction ratio shown in Table 2 below and annealed at the temperature shown in Table 2 below for 30 seconds to manufacture an annealed steel plate. The annealed steel plate is cold-rolled a second time at the reduction ratio shown in Table 2 below to manufacture a second cold-rolled steel plate. Ni plating is uniformly performed on the second cold-rolled steel plate at a thickness of 1.5 μm, and then alloying annealing is performed at the temperature shown in Table 2 below for 30 seconds to manufacture the final plated steel plate.

[0088] [Table 1]

[0089]

[0090] The remaining components in Table 1 are Fe and inevitable impurities.

[0091] [Table 2]

[0092]

[0093] For each of the steel plates manufactured at this time, calculate or evaluate the composition index, ASTM grain size number, average carbide size, alloy layer thickness, tensile physical properties at room temperature and 600 °C, coating adhesion, and shape freezing property, and show the results in Table 3 and Table 4 below.

[0094] In addition, the ASTM grain size number is an index related to the grain size, which is calculated and measured according to the ASTM E112 standard (Standard Test Methods for Determining Average Grain Size) based on the image obtained through optical microstructure observation. The larger the ASTM grain size number, the smaller the average grain size.

[0095] In addition, the equivalent circle average carbide size refers to the size of the average carbide confirmed during the microstructure observation of the finally manufactured material. At this time, the carbide refers to all carbides confirmed through ordinary microstructure observation, including all cementite phases, pearlite phases, etc., and it is impossible to distinguish according to the phase difference.

[0096] In addition, the thickness of the alloy layer refers to the thickness of the composition change layer in which Fe and Ni components coexist by diffusion between the steel plate and the Ni coating, and the cross-section of the coated steel plate can be measured by a Glow Discharge Spectrometer (GDS) or an Energy Disperse X-ray Spectrometer (EDS). Generally, for the composition of the alloy layer, the Fe content is high in the inner part close to the steel plate, and the Ni content is high in the outer part close to the coating. In the present invention, in terms of weight%, the thickness of the alloy layer is defined as the length from the position where Fe is 5% to the position where Ni is 5%. This is because it is difficult to determine the accurate position when each position is defined as the position where the composition is 0%, and measurement errors are likely to occur.

[0097] In addition, the yield strength and elongation at room temperature are measured by a tensile test at room temperature. The room temperature tensile specimen is stretched at a speed of 10 mm per minute using the JIS-5 standard to measure the physical properties. The yield strength at 600 °C is measured by a high-temperature tensile test, in which the specimen is held at 600 °C for 10 minutes and then stretched at a speed of 0.04 mm / second. During the high-temperature tensile test, an ASTM E8 plate-shaped standard specimen with a width of 12.5 mm is used, and the temperature is raised to 600 °C at a heating rate of 5 - 10 °C / second. Then, after holding for 10 minutes, it is stretched to ensure the uniformity of the specimen temperature and atmosphere.

[0098] The plating adhesion indicates the degree of bonding between the coating and the steel plate, and is evaluated by observing the surface through an optical microscope after the cylindrical battery case is formed. When observing after forming, if cracks of 10 μm or more exist in the surface part, it is judged that the corrosion resistance function of the coating has been lost, so the plating adhesion is judged to be poor.

[0099] Shape freezing property is an index indicating the degree to which the shape remains unchanged and is maintained after being formed into a structure at normal temperature. It can be evaluated by measuring the shape difference of a cylindrical battery case after forming and after the battery is manufactured and charged / discharged more than once. When the shape difference is more than 0.3 mm after forming and after manufacturing and charging / discharging, it is judged that the shape freezing property is poor.

[0100] In addition, during manufacturing, the shape difference of the steel plate, cracks in the steel plate generated during the forming of the battery case, the shape difference of the cross-section during shearing, and insufficient corrosion resistance after plating are further shown in the remarks. In the case of corrosion resistance, when rust is observed through a salt spray test (SST, Salt Spray Test) of spraying a 5 wt% NaCl aqueous solution at a temperature of 35 °C and a relative humidity of 95% for 6 hours, it is judged that the standard is not met. In addition, when burrs of more than 0.3 μm are generated during shearing, the shearing shape is judged to be poor.

[0101] [Table 3]

[0102]

[0103] [Table 4]

[0104]

[0105] As shown in Tables 1 to 4 above, Invention Steels 1 to 21 all meet the composition contents and manufacturing conditions proposed by the present invention. When the composition index is 0.85 or less, the carbide size is 1.0 - 2.0 μm, and the thickness of the alloy layer is 0.6 - 1.4 μm as proposed by the present inventor, the yield strength at normal temperature is 320 - 400 MPa, the elongation at normal temperature is 30% or more, and the yield strength at 600 °C is 80 MPa or more. It has good mechanical properties, good plating adhesion, and good shape freezing property, so it can be confirmed that the required characteristics as a material for a cylindrical battery case are met. Specifically, Invention Steels 1 to 3 are examples showing the influence of the change in the C content, Invention Steels 4 to 5 are examples showing the influence of the change in the Mn content, Invention Steels 6 to 7 are examples showing the influence of the change in the coiling temperature, Invention Steels 8 to 10 are examples showing the influence of the change in the cold rolling reduction rate in one pass, Invention Steels 11 to 13 are examples showing the influence of the change in the annealing temperature, Invention Steels 14 to 16 are examples showing the influence of the change in the cold rolling reduction rate in two passes, and Invention Steels 17 to 19 are examples showing the influence of the change in the alloying temperature. It can be seen that all the required characteristics are met within the range of the manufacturing conditions proposed by the present inventor.

[0106] In contrast, when the carbon content of Comparative Steel 1 is as low as less than 0.02%, the ASTM grain size number of the grains is less than 11, the grains are relatively large, and the carbides formed are as small as less than 1.0. Due to the relatively large grains formed, the yield strength at room temperature is less than 320 MPa, and the yield strength at 600 °C is as low as less than 80 MPa. Small carbides are formed, so the shear shape is poor. When the carbon content of Comparative Steel 2 exceeds 0.06%, the composition index exceeds 0.85, the strength at room temperature exceeds 400 MPa, the elongation is as low as less than 30%, and large carbides with a size of 2.0 μm or more are formed, and cracks are generated during processing.

[0107] When the manganese content of Comparative Steel 3 is as low as less than 0.1%, cracks occur at the edges of the hot-rolled steel sheets during hot rolling. Since solid-solution sulfur may cause brittleness at high temperatures, when it precipitates in the form of MnS by combining with Mn, brittleness may be inhibited. In the case of this example, the manganese content is low and cannot precipitate sufficiently, so cracks are generated.

[0108] When the composition index of Comparative Steel 4 to Comparative Steel 6 exceeds 0.85, the yield strength at room temperature exceeds 400 MPa, and the elongation does not reach 30%, so cracks are generated during processing.

[0109] When the chromium content of Comparative Steel 7 to Comparative Steel 8 exceeds 0.03%, uncoated areas occur on the surface, so the coating surface is poor. Therefore, the steel sheet exposed when uncoated may rust, so it is difficult to ensure corrosion resistance.

[0110] When the coiling temperature of Comparative Steel 9 is as low as lower than 580 °C, the ASTM grain size number of the grains exceeds 11.8 and they are formed small. The yield strength at 600 °C exceeds 90 MPa, which is beneficial for ensuring safety. However, the average carbide size is formed small and less than 1.0 μm, the shear shape is poor, and the strength at room temperature exceeds 400 MPa, which may cause serious die wear, so it is not preferred. On the other hand, when the coiling temperature of Comparative Steel 10 exceeds 720 °C, the ASTM grain size number of the grains is less than 11.0 and they are formed coarsely, and the average carbide size exceeds 2.0 μm and is formed relatively large. As a result, the yield strength at 600 °C is less than 80 MPa and insufficient, and the elongation is less than 30%, and cracks are generated during the forming of the battery case.

[0111] When the reduction ratio of the first cold rolling of Comparative Steel 11 is as low as less than 78%, grain nucleation does not proceed smoothly, and the ASTM grain number of the grains is less than 11.0 and they are formed coarsely. As a result, the yield strength at room temperature is as low as less than 320 MPa, the shape freezing property is poor, and the yield strength at 600 °C is low, so it is difficult to ensure safety at high temperatures. When the reduction ratio of the first cold rolling of Comparative Steel 12 exceeds 90%, the shape of the cold-rolled steel sheet is poor, and it is difficult to enter the annealing process as the next process.

[0112] Comparative steel 13 is a case where the annealing temperature of the cold-rolled steel sheet for the first time is as low as less than 630 °C. The cold-rolled steel sheet for the first time does not recrystallize during the annealing process and recrystallizes during the alloying annealing process. The object of the present invention is to obtain a high-temperature accelerated aging effect with increased strength while fixing carbon in dislocations at a high temperature during alloying annealing. However, when the cold-rolled steel sheet for the first time does not recrystallize during annealing, it is difficult to obtain such an effect. As a result, the room-temperature yield strength is less than 250 MPa and is quite low, so the shape freezing property is poor. In addition, while recrystallization occurs during alloying annealing, the diffusion rate is fast, and the thickness of the alloy layer exceeds 1.4 μm and is formed relatively thick. As a result, Fe is exposed on the surface of the coating and the corrosion resistance is also poor. On the other hand, in comparative steel 14, the annealing temperature of the cold-rolled steel sheet for the first time exceeds 810 °C, and the ASTM grain size number of the grain diameter is less than 11.0 and is formed relatively large. As a result, the yield strength at 600 °C is as low as less than 80 MPa, and the safety at high temperatures is reduced.

[0113] Comparative steels 15 to 18 are cases where the alloying index defined by relational expression 2 satisfies the range of 2.1 to 2.6. When the alloying index is less than 2.1, the thickness of the alloy layer is too thin and the coating adhesion is poor. When the alloying index exceeds 2.6, the thickness of the alloy layer is too thick and the corrosion resistance is poor. Specifically, in Comparative Example 15, the temperature is as low as less than 650 °C. Even with a long annealing time of 30 seconds, the thickness of the alloy layer is formed too thin. When annealing for a long time of more than 30 seconds, the manufacturing time is long, and there is a disadvantage of low productivity. In Comparative Example 18, the temperature exceeds 700 °C. Even when the annealing time is as short as 10 seconds, the thickness of the alloy layer is formed too thick. When the annealing time is short, it is difficult to control at a high temperature, and there is a disadvantage of an increased deviation.

[0114] In comparative steel 19, the reduction ratio of the second cold rolling is less than 0.8%, and the shape after the second cold rolling is poor. This is because the reduction ratio of the second cold rolling is low, and it is difficult to sufficiently obtain the effect of improving the shape of the steel sheet with partially deformed during annealing at a high temperature. Comparative steel 20 is a case where the reduction ratio of the second cold rolling exceeds 1.8%. Due to the work hardening effect during cold rolling, the room-temperature yield strength exceeds 400 MPa, causing serious die wear, so it is not preferred.

[0115] As described above, the preferred embodiments of the present invention have been described in the detailed description of the present invention. However, those skilled in the art can make various deformations without departing from the scope of the present invention. Therefore, the scope of the rights of the present invention is not limited to the described embodiments and should be determined by the claims and their equivalents.

Claims

1. A Ni-plated steel sheet for cans, by weight%, the Ni-plated steel sheet for cans comprises: C: 0.020 - 0.060%, Si: 0.050% or less, Mn: 0.1 - 0.6%, Al: 0.010 - 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.006% or less, Cr: less than 0.030%, the balance being Fe and other inevitable impurities, and the composition index represented by the following relational expression 1 is 0.85 or less. The ASTM grain size number satisfies 11.00 to 11.

70. The Ni-plated steel sheet for cans has a fine steel microstructure containing carbides with an equivalent circle average size satisfying 1.0 - 2.0 μm. A Fe-Ni alloy layer with a thickness of 0.6 - 1.4 μm is formed on the surface of the Ni-plated steel sheet for cans. [Relational expression 1] Composition index = C × 10 + Mn Among them, C and Mn respectively represent the weight% of C and Mn.

2. The Ni-plated steel sheet for cans according to claim 1, wherein, The plated steel sheet has a fine microstructure containing 90% or more ferrite and 10% or less cementite by area%.

3. The Ni-plated steel sheet for cans according to claim 1, wherein, The normal temperature yield strength of the Ni-plated steel sheet for cans satisfies 320 - 400 MPa, the normal temperature elongation satisfies 30% or more, and the yield strength at 600 °C satisfies 80 MPa or more.

4. A manufacturing method of a Ni-plated steel sheet for cans, which comprises the following steps: Reheat the steel billet to 1150 °C or higher. By weight%, the steel billet comprises: C: 0.020 - 0.060%, Si: 0.050% or less, Mn: 0.1 - 0.6%, Al: 0.010 - 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.006% or less, Cr: less than 0.030%, the balance being Fe and other inevitable impurities, and the composition index represented by the following relational expression 1 satisfies 0.85 or less; Hot finish roll the reheated steel billet above Ar3 to manufacture a hot-rolled steel sheet. After cooling the hot-rolled steel sheet, coil it at a temperature of 580 - 720 °C. Cold roll the coiled hot-rolled steel sheet at a reduction ratio of 78 - 90% to manufacture a first cold-rolled steel sheet. Anneal the first cold-rolled steel sheet at a temperature of 630 - 810 °C. Cold roll the annealed steel sheet at a reduction ratio of 0.6 - 1.2% to manufacture a second cold-rolled steel sheet. Electroplate Ni on the second cold-rolled steel sheet to manufacture a Ni-plated steel sheet; and Alloy anneal the Ni-plated steel sheet at a temperature of 650 - 700 °C for a time when the alloying index defined in the following relational expression 2 satisfies 2.10 to 2.60, thereby forming a Fe-Ni alloy layer with a thickness of 0.6 - 1.4 μm on the surface of the cold-rolled steel sheet. [Relational expression 1] Composition index = C × 10 + Mn wherein, C and Mn respectively represent the weight% of C and Mn. [Relational expression 2] Alloying index = (2 - (700 - T) / 100) × t 0.1 wherein, T represents the temperature, the unit of which is °C, and t represents the time, the unit of which is seconds.

5. The manufacturing method of the Ni-plated steel sheet for cans according to claim 4, wherein, During the Ni electroplating, Ni electroplating is carried out in a thickness range of 0.6 - 5.0 μm.

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